Our plan is to study the kinetics of the development of anti-tumor killer cells and regulatory cells in T cell populations and to determine the nature of interactions amongst T cell sub-populations in both the generation of killer cells and in their attack on target cells. To do this we will inoculate various doses of normal parental thymocytes and/or various thymocyte sub-populations (fractionated by sensitivity to cortisone, to radiation, migratory characteristics, density and antigencity) into lethally irradiated F mice. At intervals thereafter we will harvest the spleens and lymph nodes of the recipient mice and test the ability of the recovered cells to kill tumor cells containing the H-2 antigens of the reciprocal parent in the F1 cross. We will test the effect of combining the sub-populations described above, both during immunization and in the killing assay. We will test the effect of passively administered antibody on these phenomena.